ADSORPTION OF CsONSi(111)-(7 × 7)SURFACES: SITE PREFERENCE AND THE NEAR-METALLIC STATE OF Cs

This study involves the adsorption of Cs onSi(111)-(7 × 7)surfaces at room and elevated temperatures, with increasing coverage of Cs to saturation, in ultrahigh vacuum (UHV) conditions. The techniques of low energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and work function (WF) measurements were utilized. A WF change at the minimum, found to beΔϕ=-3.2eV at an approximate 0.23 ML Cs coverage, corresponds to half the saturation coverage. We propose that, at that coverage, the adatom dangling bonds of theSi(111)-(7 × 7)substrate are completely filled. At saturation coverage (0.47 ML), Cs forms a single saturation layer in a near-metallic surface state, where the atomic radius of the Cs adatoms is likely to be2.2× 10-8cm. The surface exhibits a high degree of disorder with 0.47 ML of adsorbed Cs, and it is likely that the adsorbate remains in cluster-like domains within the center of the disordered7×7unit cells, instead of evenly covering the Si surface. Saturation coverage of Cs onSi(111)-(7 × 7)surfaces exhibits a WF value 0.5 eV less than that of pure metallic Cs. Cesium adatoms form a stronger bond to theSi(111)-(7 × 7)surface than to Ni(100), with a calculated energy of 1.64 eV/atom. The induced surface disorder and strong binding energy are evidence of the strong Cs–Si interaction, which most likely prevents the formation of a purely metallic Cs overlayer.